1. Technical Field
This invention is directed towards regenerative heat pump system and method using a working fluid or refrigerant and an adsorbent material.
2. Discussion of the Invention
Heat pumps using solid adsorbent beds are known as shown by U.S. Pat. No. 4,610,148, U.S. Pat. No. 4,637,218 and U.S. Pat. No. 4,694,659 which use zeolite as the adsorbent and water as the working fluid. In general since adsorbents take up the working fluid when cooled and desorb the working fluid when heated, adsorbent heat pumps are said to be heat driven. Often in adsorbent heat pumps two beds of sorbents are used, one to adsorb the working fluid while the other bed is desorbing the working fluid. Alternate heating and cooling of the beds is the conventional procedure. When used in air conditioning, heat from an interior room may be used to evaporate the working fluid in an evaporator with heat rejection to the environment at ambient temperatures.
In all of such systems the efficiency of the apparatus is measured by its coefficient of performance or "COP". By the term "COP" as used herein is meant the ratio of heating or cooling work performed divided by the amount of power required to do the work. Since cooling COP.sub.S, or COP.sub.CS, are generally lower than heating COP.sub.S, or COPM.sub.S, many systems are rated on their cooling COP.sub.S.
U.S. Pat. No. 4,637,218 mentions cooling COP.sub.S between 1 and 2 and heating COP.sub.S between 2 and 3 both of which are apparently ideal values since it is later stated that in practice for the heating mode the COP is less but with proper design is nevertheless within about 80% of the theoretical value, that is, about 2.4. In U.S. Pat. No. 4,637,218 a hot coolant is pumped from a hot 204.4.degree. C. sorbent compressor to a cooler 37.8.degree. C. sorbent compressor, while at the same time cold coolant is pumped from the cooler sorbent compressor to the hotter sorbent compressor. Both compressors exchange heat yielding a typical heat regeneration efficiency of about 80%. The remainder of the heat is supplied by a boiler at about 204.degree. C. Water vapor as the working fluid is desorbed from the hot sorbent at, a relatively high pressure while the cold sorbent adsorbs the working fluid at a relatively low pressure. Expansion of the working fluid from the higher pressure to the lower pressure creates net cooling at 4.4.degree. C.
U.S. Pat. No. 4,610,148 reports a theoretical heating COP of about 3 and a cooling COP of about 2, and, a calculated operating COP.sub.H of about 2.6 and a calculated operating COP.sub.C of about 1.6.
FIG. 3 of U.S. Pat. No. 4,694,659, which is concerned with a dual sorbent bed heat pump, shows heating and cooling COP's as a function of a dimensionless thermal wavelength parameter which at a value of about 0.5 corresponds to a heating COP of about 2.7 and a cooling COP of about 1.7.
Cryogenic cooler systems for sorption refrigerators using a sorption compressor, a heating/cooling loop and a Joule-Thomson expansion valve, or "J-T" valve, with methane as a refrigerant gas and charcoal as the adsorbent, are disclosed in articles entitled "High Efficiency Sorption Refrigerator Design", and "Design and Component Test Performance of an Efficient 4 W, 130 K Sorption Refrigerator" in Advances In Cryogenic Engineering, Vol. 35, Plenum Press, New York, 1990. Desorption occurs at 4.46 MPa (646 psia), i.e. P.sub.H, and adsorption at 0.15 MPa (22 psia), i.e. P.sub.L, or a pressure ratio of about 30, i.e, P.sub.H /P.sub.L =30. Methane is expanded from 4.46 MPa to 0.15 MPa to achieve cooling below 130K (-143.degree. C.). The sorbent is heated from 240K (-33.degree. C.) to 600K (327.degree. C.) to desorb the methane. However based on data from another source it has been noted that for this cryogenic cooler system the sorbent must be heated from 240K to 415K (142.degree. C.) before any methane is desorbed, and the sorbent must be cooled from 600K (327.degree. C.) to 320K (47.degree. C.) before any methane is adsorbed. The temperature ranges with high thermal capacitance, which accounts for the heat of adsorption, during heating is from 415K to 800K, and during cooling is from 320K to 240K. Since the heating high thermal capacitance temperature range does not overlap the cooling high thermal capacitance temperature range, none of the heat of adsorption can be recovered for use in the system, and as a consequence the heat of adsorption must be rejected entirely from the system. In the above mentioned methane/charcoal cryogenic refrigerator, the system's high pressure ratio of about 30 essentially precludes regeneration of the heat of adsorption. Usually in systems with high pressure ratios, i.e. P.sub.H /P.sub.L ratios over about 10, none of the heat of adsorption can be regenerated. By the term "sensible heat" as used herein is meant the "mass" times "specific heat" times "temperature change". Therefore unless otherwise specified, the term "sensible heat" as used herein does not include latent heat or heat of adsorption.
In order to improve system efficiency and COP.sub.S it therefore would be desirable to regenerate at least a portion of the heat of adsorption.